The National Security Agency has admitted that some of its analysts deliberately abused its surveillance systems, with one analyst disciplined for using NSA resources to track a former spouse.

The agency said Friday it had found "very rare instances of wilful violations of NSA's authorities" as officials briefed reporters that various agents had used the NSA's controversial data monitoring capabilities to spy on love interests.

"NSA takes very seriously allegations of misconduct, and co-operates fully with any investigations – responding as appropriate," the NSA said in a statement. "NSA has zero tolerance for willful violations of the agency's authorities."

It said none of the abuses involved violations of the Foreign Intelligence Surveillance Act or the Patriot Act – violations of which have been highlighted by the Guardian based on documents leaked by the whistleblower Edward Snowden. Instead, the abuses were related to misuse of the 1981 Executive Order 12333, which governs how US intelligence operations are used.

The Bloomberg news agency reported that anonymous US officials had said there had been "a few cases" where NSA officials or contractors had used agency surveillance tools or data to spy on people in which they had romantic interests.

The Wall Street Journal also said anonymous officials had admitted that NSA analysts had abused their positions to monitor love interests. It said the practice is infrequent but "common enough to garner its own spycraft label: LOVEINT". The newspaper said that NSA employees or contractors found to have committed LOVEINT violations had been disciplined in each case.

In its official statement, the NSA did not directly address the issue of data monitoring for amorous purposes. The agency admitted that abuses had taken place over the past decade but did not specify what the nature of those abuses were.

The Senate intelligence committee was briefed this week on the "wilful violations" by the NSA's inspector general's office. Senator Dianne Feinstein, who chairs the committee, issued a statement on the abuses.

"The committee has learned that in isolated cases over the past decade, a very small number of NSA personnel have violated NSA procedures – in roughly one case per year," Feinstein said.

She said the incidents "in most instances did not involve an American's information".

"I have been informed by NSA that disciplinary action has been taken, and I am reviewing each of these incidents in detail."

Last week the NSA's director of compliance, John DeLong, said abuses "are taken very seriously."

"When we make mistakes, we detect, we correct and we report," he said.

Obama administration officials and intelligence overseers in Congress have described the Fisa and Patriot Act violations as inadvertent. The NSA this week declassified a secret Fisa court ruling from 2011 that revealed the agency had inadvertently scooped up, over a three-year period, as many as 56,000 emails of Americans not connected to terrorism.